1. Field of the Invention
This invention is in the field of autofocus methods for search (swath) Synthetic Aperture Radar (SAR) imaging where missing pulses are encountered.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a continuous sequence of transmitted pulses. These transmitted pulses are from a relatively small antenna on a moving platform. As the platform moves, the information reflected from the sequence of pulses is coherently combined to arrive at a high resolution SAR image.
The plurality of sequential returns creating a SAR image generated by the transmitted pulses along a presumed known path of the platform make up an array. Theoretically, during the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.
The clarity of a SAR image is in many respects dependent on the quality of the motion compensation applied to each radar return prior to SAR image computation. Motion compensation shifts the phase of each radar sample (typically an I+jQ complex quantity derived from an analog to digital converter) in accordance with the motion in space of the moving platform with respect to a reference point. The SAR imaging process depends on the coherent, phase accurate summing of the sequence of all radar returns expected within an array.
For certain applications the accuracy of the motion compensation derived phase compensation applied to each radar A/D sample is insufficiently accurate. For better phase alignment accuracy autofocus (AF) methods are used. Autofocus (AF) methods typically use information contained in the radar returns of the SAR data itself in an attempt to phase align radar return samples to accuracies better than those available from motion compensation alone. Estimated phase error derived from collected SAR data is applied to the motion compensated SAR data to improve the resulting SAR image. This SAR data driven approach for phase error estimation and compensation is generally referred to as autofocus (AF).
In addition to the spotlight mode, SAR radar can also be operated in the search (swath or strip) map mode. Spotlight mode produces two dimensional images of limited size of a limited area by steering the antenna beam to the center of the map (image) center for the duration of a frame (or array). Conversely, search mode produces image strips of theoretically unlimited length by maintaining a fixed azimuth antenna orientation during a SAR data collection period, or array. Unlike spotlight mode that typically produces images oriented in the range-azimuth direction, search mode produces images oriented in the along track and cross track direction. Uncompensated platform motion during search mode results in image smearing in the azimuth direction caused by pulse data that affects azimuth response. Because target image smears in the direction of both image axes, it is difficult to estimate and correct residual phase error for autofocus using one dimensional batch processing.
Another difficulty presented during search mode comes from incomplete arrays. That is, a number of radar pulse returns forming the sequence of returns in an array may be missing. The missing pulses within an array blur the resulting SAR image, obfuscating details thus rendering it of limited utility, necessitating restoration of the missing pulse returns. However, restoration of missing pulses is complicated by the need to orient the data in the Range and Azimuth direction, presenting a large computational load on the on-board processor.